This invention relates generally to a catalytic process capable of selectively producing a desired product from a given reactant and is particularly directed to the use of catalysts in the form of microporous crystalline solids, commonly referred to as molecular sieves.
The trend today in catalysis research is toward the development of the so-called "designer" catalysts, i.e., catalysts that will selectively produce the desired product from a given reactant. The class of materials that offers the greatest hope for designer catalysts is that which includes all the various types of molecular sieves. Molecular sieves are microporous crystalline solids composed of interconnecting channels and cavities. The microporous structure of molecular sieves provides a large, well defined internal surface area for catalysis or absorption and causes molecular sieves to be shape selective. Many types of molecular sieves with varying structures, pore sizes, combinations of framework atoms, and framework atom ratios have been either discovered or synthesized.
Included among the properties that cause molecular sieves to be promising for industrial applications are:
1. well defined crystal structure. PA1 2. high internal surface area. PA1 3. uniform pores with one or more discrete sizes. PA1 4. good thermal stability. PA1 5. exchangeable cations which can be used for catalysis; and PA1 6. exchangeable framework metal atom chemistries.
Molecular sieves are classified according to the nature of their principal framework atoms, the ratio of these atoms, any substituent atoms present, and their pore size. The first three classifications determine the catalytic activity of the molecular sieve, while the pore size is important to the shape selectivity of the sieve.
There are three basic types of shape selectivity in molecular sieves. The first is reactant selectivity. If a reactant will not fit into the molecular sieve's pores, it cannot be catalyzed or absorbed by the internal sites. (It can, however, react with sites on the surface, but at a greatly reduced reactivity level because of the reduced number of such sites.) The second type of molecular sieve shape selectivity is transition state selectivity. If a reaction proceeds through a transition state that is too large to form within the molecular sieve's pores, the reaction cannot take place within the sieve. Finally, there is product selectivity. Any product formed must be small enough to diffuse out of the molecular sieve. Such shape selectivity constraints are very important to the designing of new catalysts for specific chemical synthesis.
Molecular sieves are usually synthesized by crystallization from a gel containing the desired framework atoms and a template, i.e., a molecule or molecular cation, usually organic, which determines the crystalline structure and pore size. The gel composition determines the framework atoms and their ratios. The template directs the crystallization process towards the desired sieve structures. After formation, the template is burned (calcined) away leaving an open pore structure.
The most well known class of molecular sieves is the zeolites. Zeolites are composed of silicon and aluminum atoms bridged by oxygen atoms in a tetrahedral arrangement. Zeolites have many different Si/Al atom ratios and pore sizes. Their catalytic activity is due to a charge imbalance at any Al-O-Si bonds. This imbalance creates negative charge in the framework which must be compensated by cations. When this cation is a proton, a Bronsted acid site is formed. Such sites are thought to be the main source of catalytic activity in the sieves, though other types of sites have been theorized. It should be noted that the sieve structure terminates in O--H bonds. Some of these exterior sites may catalyze reactions and are not subject to selectivity restrictions. However, these sites are generally not as reactive or as numerous as the intracrystalline Bronsted sites and, therefore, have only a small effect on reaction chemistry.
Another class of molecular sieves is the aluminophosphates (AlPO.sub.4 's). These types of sieves, which are important to the present invention, are composed of aluminum(III) and phosphorous(V) atoms bridged by oxygen atoms and have P(V)/Al(III).apprxeq. 1. However, AlPO.sub.4 frameworks are neutrally charged and, therefore, exhibit little or no catalytic behavior. To create catalytic Bronsted sites in the AlPO.sub.4 's, the Al(III) and/or P(V) positions are doped with a differently charged element, such as magnesium(II), manganese(II), cobalt(II), iron(II), zinc(II) or silicon(IV), where the divalent metals substitute for Al(III) while the silicon substitutes for P(V) in the AlPO.sub.4 framework. These substitutions produce a negatively charged framework and Bronsted acid sites (see FIG. 1).
It has recently been reported that when Co(II) is substituted for Al(III) in the framework of certain aluminophosphate (AlPO.sub.4) molecular sieves and the resulting Co(II)containing AlPO.sub.4 (CoAPO) is calcined in oxygen, the Co(II) is oxidized to Co(III). Further work with these Co(III) CoAPOs showed that they possess strong oxidizing capability and, for example, can convert methanol to formaldehyde (at 25.degree. C.), NO to NO.sup.+ (at 25.degree. C.), and H.sub.2 to 2H.sup.+ (at .gtoreq. 300.degree. C.). See L. E. Iton, I. Choi, J. A. Desjardins and V. A. Maroni, Zeolites 9, 535 (1989).
The nature of the present invention is (1) the discovery that certain transition metal-substituted aluminophosphate (AlPO.sub.4) molecular sieves can activate methane gas (CH.sub.4) to produce C.sub.2+ hydrocarbons at temperatures of 500.degree. C. or less, (2) that the catalytic activity of these catalysts can be maintained using a variety of chemical and electrochemical methods, and (3) that product selectivity can be controlled by judicious selection of sieve pore structure and by integrated use of various combinations of molecular sieve materials.